A polypropylene resin composition has been known to be difficult to be paint-coated, printed and bound due to their non-polarity. Therefore, there has been proposed an improved method for paint-coating the molded article of a polypropylene resin composition. For example, so-called a primer or an under-coating is usually applied as a inter-layer between the polypropylene resin composition and the paint-coated layer to improve their compatibility. This method is, however, still unsatisfactory from the industrial view point because a primer itself is expensive, an additional step required to apply a primer increases the cost of the final product, and a primer causes an environmental problem associated with its waste treatment.
Furthermore, the application of a primer is usually performed by cleaning with vapor of halogenated hydrocarbon such as 1,1,1-trichloroethane. But, the use of the halogenated hydrocarbon is now being banned in consideration of the ozone layer depletion or the protection of the global environment.
The another method for improvement of the surface treatment of the molded articles of a polypropylene composition is proposed to increase the bonding strength of paints, prints or adhesives to them.
For example, sandblasting, chromic acid treatment, flame treatment, corona discharge treatment, plasma treatment, surface activation with functional groups, and photo-induced surface grafting are applied before paint-coating the molded article. However, none of them is satisfactory from the standpoint of performance and environmental problem.
Under these circumstances, the present inventors conducted a series of researches with respect to the methods for surface treating and paint-coating a polypropylene resin composition molded article, which led to the finding that the surface property of the molded article is greatly improved when the surface of a specified polypropylene resin composition molded article is irradiated with ultraviolet light having a specified irradiation wavelength, and attained the present invention.